The present invention relates to a hybrid Rankine cycle system.
A typical power plant incorporating a Rankine cycle system basically has a boiler B, a steam turbine ST, a condenser C and a feedwater pump FP. The boiler B is generally equipped with a superheater SH.
The boiler B heats a working medium or water to generate vapor or steam. The saturated steam of high temperature and pressure generated in the boiler B flows into a superheater SH and is superheated to become superheated steam of higher temperature. The high-pressure steam from the superheater SH expands through the steam turbine ST to produce mechanical work, and is then discharged with relatively lower temperature and pressure. The mechanical work thus produced in the steam turbine ST is converted into electrical power by means of a generator G connected to the steam turbine ST. The steam from the steam turbine ST passes through the condenser, where it condenses into condensate CD on heat exchange with cooling water CW supplied from the outside. The condensate CD from the condenser C is pumped by a feedwater pump FP to the boiler B to complete the cycle.
In general, the thermal efficiency .eta. of the power plant gauges the extent to which the energy input to the working fluid flowing through the boiler is converted to the net work output. Thus, in a basic cycle, the thermal efficiency .eta. is represented by the following formula: EQU .eta.=(W.sub.turbine -W.sub.pump)/Qinput
where, W.sub.turbine represents the work done outside by the steam turbine ST, W.sub.pump represents the work input to the feedwater pump FP and Q.sub.input represents the energy input to the boiler.
The following two ways can be available for improving the thermal efficiency:
(1) Increase temperature and pressure of steam to be supplied to the steam turbine ST, and PA1 (2) Reduce temperature and pressure of the steam discharged from the steam turbine ST. PA1 means including a heating device and for separating working medium vapor from a weak absorbent solution including said working medium and a substance having boiling point higher than the heating temperature in the heating device, whereby the substance may remain therein as a strong absorbent solution; PA1 a vapor driven prime mover through which said vapor from the vapor separating means expands to produce work outside; PA1 an absorber condenser means for introducing thereinto the strong absorbent solution to absorb the vapor from the prime mover to produce the weak absorbent solution; PA1 means for delivering the weak absorbent solution towards the vapor separating means; and PA1 means for delivering the strong absorbent solution from the separating means towards the absorber condenser means.
Referring to the first method, the maximum temperature and pressure are limited to be between 811.degree. K and 839.degree. K and to be not higher than 2.46.times.10.sup.2 Pa, respectively, in the technical point of view of heat resisting strength of the boiler material. Accordingly, it is difficult to expect the further improvement in the thermal efficiency due to increase in the temperature and pressure.
The temperature and pressure of the steam discharged from the steam turbine depends on the temperature of the cooling water CW. In general, the pressure of the steam from the steam turbine corresponds to a saturation pressure of steam at a temperature higher by 5.degree. C. to 10.degree. C. than that of the cooling water CW. The temperature of the steam from the steam turbine corresponds to a temperature to which the steam passing into the steam turbine reaches when such steam expands to reduce the pressure thereof into that of the steam leaving the steam turbine. In consequence, the improvement in the thermal efficiency by the second method requires cooling water of a lower temperature and, hence, is limited undesirably.
On the other hand, a Karina cycle is known which does not require cooling water of low temperature. The Karina cycle makes use of ammonia as a working medium for a vapor prime mover. The working medium (ammonia) is absorbed by water so that the temperature and pressure of vapor from the prime mover are lowered. The Karina cycle, however, requires various additional safety measures because of combustibility and toxicity of ammonia. Ammonia of 0.5%-1% concentration in terms of volumetric ratio causes a fatal effect within 30 minutes. Thus, the Karina cycle is not suitable to practically carry out and requires a complicated arrangement.